Fungal polyketides are secondary metabolites synthesized by molds through a head-to-tail assembly of acetate units. Among them are aflatoxins and related compounds, which have been identified as widespread mycotoxins responsible for high incidence of human cancer, a large number of animal deaths, and significant loss in agricultural productivity. In this proposed research, the biosynthetic mechanism of aflatoxins will be used as a model to elucidate biochemical events involved in the synthesis of fungal polyketides and to develop ways to control or prevent toxigenesis. Methods include use of deficient mutants or enzyme inhibitors that block specific steps in the biosynthetic pathway to accumulate intermediates, 14C and 13C tracing to establish precursor/product relationships, and development of cell-free systems for enzymatic studies. The results will provide ways to produce various aflatoxins, including aflatoxin M1, and their biogenetic intermediates which can be used as analytical standards or sources of potential antitumor drugs. Thus the objectives of the proposed research are to better the understanding of the fungal secondary metabolism as related to toxigenesis, to expedite the control of toxic polyketides as food-borne mycotoxins, and to explore the therapeutical properties of the biogenetic intermediates of aflatoxin biosynthesis.